It is well known in the art of multilevel interconnection that several wiring layers are required to connect together the devices available on a very large scale integrated (VLSI) chip into a usable circuit. The interconnection structure consists of electrically conducting fine wires of at least one layer separated by dielectric insulating films. The interconnection structure, comprising thin wires embedded in the insulator, is similar to a transmission line, in that there is a propagation delay of the signals being transmitted. This delay, often referred to as RC delay (R=resistance; C=capacitance), has become a significant part of the overall delay of the circuit in fast switching digital devices.
One approach has been to use organic dielectrics with lower dielectric constant than conventional inorganic insulating materials in order to minimize the capacitance term in the RC delay. The most commonly used dielectric thin film in the art has been SiO.sub.2, which has a dielectric constant, or a relative permittivity, of 4.0. Organic polymers, such as polyimides, have dielectric constants lower than 4.0 and thus are very attractive candidates for dielectric films for multilevel interconnection. In spite of the availability of a variety of polyimides, their use as multilevel insulators has been limited because of some unfavorable characteristics of the organic insulators. One of two major limitations was that if standard reactive ion etching (RIE) was used to define a conventional aluminum conductor, the open structure of the organic insulator absorbed a large amount of Cl.sub.2 from the RIE plasma, which caused corrosion of the aluminum lines. The second limitation was that the organic insulator is never 100% cured in spite of all precautions, leaving trace amounts of volatile components that tend to desorb or outgas on subsequent heating.
In one application that has been used for relatively large feature sizes, R. M. Geffken, "Multi-Level Metallurgy for Master Image Structured Logic," IEDM 1983 Proceedings, pp. 542-545, an organic dielectric was used as an insulator for interconnection, a metal interconnection layer was patterned by a lift-off technique. The resist was formed by conventional double exposure to have a negative (undercut) profile suitable for lift-off definition of the metal pattern. The etching of patterns in the organic insulator, mostly vias, are done for large dimension apertures by wet etching before curing and for smaller dimensions by reactive etching with sloped resist. In the lift-off metal process, several of the organic layers are in contact together and the outgassing component freely escaped through the layers. However, since metal RIE is needed to define a conductor pattern, an impermeable layer had to be used to prevent the absorption of unwanted etching gases, such as chlorine, from the reactive ion etching process. The organic insulators are carefully cured and baked and sealed with an inorganic thin film such as silicon nitride to seal all of polyimide surface from the etching gases. In this method, H. Eggers et al., "A Polyimide-Isolated Three-Layer Metallization System", IEEE V-MIC Conf. Proceedings, 1985, pp. 163-169, sloped via surfaces of the polyimides are also coated with the inorganic insulator. A great deal of precaution has to be taken in order to avoid defects or trapped outgassing material, as these can affect the reliability of the structure during later processing and use.
Both of these techniques, lift-off metal patterning and sloped vias, are more or less satisfactory for coarse feature sizes, but have become difficult to extend to the finer dimensions required by the shrinking of component size and the increase in the number of circuits on a chip. The new requirements are for fine wires and spaces (1 micron and less) and for vertical vias connecting wires between levels. These dimensions in turn need the use of metal patterning techniques such as reactive ion etching and metal patterning by chemical mechanical polishing (metal damascene) as shown in U.S. Pat. No. 4,954,142.
Another design requirement for high density wiring is the use of partially intersecting vias and wires. According to this requirement, the via opening (sometime referred to as a stud or pillar because of its vertical profile) and the metal wire over the via need to make contact only over a portion of the common surface. This means that, if reactive ion etching is used to define vias, any over etching will etch deeper holes into the insulator below the via level. This effect leads to problems in subsequent filling of the via hole by metal deposition processes such as chemical vapor deposition, hot sputtering etc. An etch stop layer present between the two dielectric layers will provide the cushion for overetching, and will minimize the unwanted etching in the insulator below. This is an important requirement for ultra large scale integration to allow partially overlapping connections.
In addition to protection from the chemicals used in metal patterning, protection of the organic layer below while etching the organic layer above is also required. One of the ways to meet this additional requirement is by use of photosensitive polyimide. U.S. Pat. No. 5,091,289 illustrates a prior art circuit in which a photosensitive organic layer is patterned and converted to a patterned organic dielectric layer. The layer below is made non photosensitive by a prior process and hence the need to protect the layer below is avoided. However, photosensitive polyimides shrink considerably on curing from the loss of photoactive components, resulting in sloped vias, which is undesirable for ULSI wiring.
Another requirement for a metal damascene process, in which grooves in an insulator are overfilled with metal and the excess metal is removed by polishing, is that the insulator be a good polish stop. Analogous to the etching process, in the absence of a good polish stop, overpolishing will thin the insulator once the excess metal is removed and thereby lead to variable insulator thickness as well as unacceptably low insulator thickness in spots. Organic insulators have notoriously low resistance to polishing process.